In the prior art, niobium cavities are desirable for building blocks of particle accelerators, free electron lasers and the like and are well known in the art. Such niobium cavities are fabricated from high purity niobium sheet or cast plate, via deep drawing, e-beam welding and chemical surface cleaning to obtain high accelerating gradients and quality factors. Often the quality factors at high gradients degrade over time for cavities produced by these methods. Such degradation appears to be affected by adherent surface oxide layers, trapped hydrogen and/or interactions between interstitial oxygen and hydrogen in the niobium material. The release of oxygen, hydrogen or the reaction products of these materials results in degradation of the vacuum within the niobium cavities thereby negatively affecting the quality of the output of such cavities.
Thermal treatment of a cavity is useful for obtaining good performance and removing impurities. However, even with thermal treatment under vacuum the cavity surface absorbs residual gas species during cool down to ambient temperatures in a vacuum furnace. Typically a chemical treatment is used following the heat treatment to recover performance. However, as typically performed in the prior art, the chemical treatment has drawbacks as it affords the reintroduction of hydrogen into the cavity and the formation of natural oxides of several nanometers in thickness.
A method of passivating niobium cavities to reduce the negative effects of these gases and impurities is described in U.S. Pat. No. 7,151,347 incorporated herein by reference herein in its entirety. The U.S. Pat. No. 7,151,347 patent describes a multi-step process that includes among others a thermal treatment step and a step of sputtering a passivating layer of niobium nitride onto the surface of the cavity. As the niobium cavity has a complex three dimensional structure with multiple surfaces with varying degrees of accessibility to the sputtering, obtaining a uniform deposition of a layer of niobium nitride over all of the surfaces is difficult utilizing sputtering. Further the depth of the layer of niobium nitride is relevant to the efficacy of the passivacation, e.g. essentially complete coverage is desired but excess deposition impacts the thermal performance of the cavity. Obtaining a uniform, thin layer over all the surface of the cavity is challenging utilizing sputtering.